1. Field of the Invention
The present invention relates to an adaptive threshold controlled decision circuit suitable for receiving optical digital signals contaminated with ringing components.
2. Description of the Related Art
In computers and communications equipment, optical interconnection networks are used to transfer optical binary signals between LSI circuits with no format conversion. As constituent elements of the interconnection networks, attention is currently focused on DC-mode optical receivers capable of receiving binary digits of the same value which last for an indefinitely long period of time. Since the required transmission rate is more than several 100 of megabits per second, DC-mode optical receivers with a constant decision threshold and a dynamic range of 10 to 15 dB are not practical because of the difficulty to support a sufficient bandwidth to meet the speed requirement. AC-mode optical receivers are also known in the art. This type of optical receivers is used in combination with a coding circuit as an optical data link to take the benefit of its excellent sensitivity characteristic to low optical input levels.
An important factor to be taken into consideration in the design of an optical receiver is the generation of noise-like components, or "ringing", caused by electrical crosstalk between optical transmitters. The ringing occurs on both high and low level laser outputs when laser diodes are driven at logical-1 (i.e., mark) and logical-0 level (i.e., space), respectively. Although the ringing that occurs at low level laser output can be reduced to a minimum by setting the bias current of laser diodes at a value sufficiently lower than their threshold level, it is impossible to eliminate the ringing that occurs at high level laser output.
As shown in FIG. 1, a prior art optical receiver disclosed in Japanese Laid-Open Patent Specification Hei-8-84160 includes a photodiode 1 for converting optical unipolar input pulses (see FIG. 1) to an electrical current signal which is fed to a differential preamplifier 2. The preamplifier produces a pair of voltage signals of opposite logic levels. The true output of the preamplifier is supplied to a peak detector 3 and an adder 5, and the complementary output is supplied to a peak detector 4 and an adder 6. The difference between the outputs of adders 5 and 6 is determined by a subtractor 7. As a result, the decision levels of a decision circuit 8, or Schmitt trigger are adaptively controlled by the input signal. When the difference output of subtractor 7 is higher than threshold V.sub.H, the output of decision circuit 8 goes high and when it reduces to a level lower than threshold V.sub.L, the output of decision circuit 8 goes low.
As shown in FIG. 2, when the optical input level is low, there is no ringing component. After two successive cycles of marks and spaces, the optical input is maintained at a high level which is rich in ringing components. If no ringing were present, the output of subtractor 7 would steadily decay and stay at a midpoint of the two threshold values, so that the optical receiver could be used as a DC-mode optical receiver. However, due to the presence of ringing component, the difference signal crosses the high and low threshold levels in rapid succession, producing error pulses. Since the amplitude of the ringing component is proportional to the optical input power, this ringing problem will become serious and the implementation of a DC-mode optical receiver operating on a high optical input level becomes difficult. On the other hand, if the optical input contains no DC components, the prior art optical receiver can still be used as an excellent AC-mode optical receiver.
Furthermore, optical receivers are used in both interconnection networks and data links and demand for such applications is enormous, there is therefore a need for dual mode optical receivers.
Japanese Laid-Open Patent Specifications Sho-62-206947 and Hei-2-266630 disclose DC-mode optical receivers immune to ringing components. However, it is impossible to modify the prior art DC-mode optical receivers into AC-mode optical receivers to be used in tandem connection while canceling offset components.